3. Barkley Marathons

This backwoods challenge takes place in Wartburg, Tennessee where participants run a 20-mile loop five times over. If it takes longer than 12 hours to complete a loop, you’re disqualified. By the end of the 100-mile journey, runners will have racked up over 54,000 feet of vertical climbing, or more than enough to threaten even the most seasoned of athletes.